One of the main Tube lines serving the Olympic Park was suspended in east London on Friday morning as hundreds of thousands of people made their way to events including the first day of athletics competition.
A signal failure in the Bethnal Green area meant the Central line was suspended eastbound between Liverpool Street and Leytonstone, Transport for London (TfL) said.
A TfL spokesman said it was working to fix the problem and had staff at Liverpool Street directing people onto other Tube lines and London Overground services to Stratford.
"We are advising spectators going to Stratford to use the wide range of other lines serving the Olympic Park, including national rail services from Liverpool Street and the District and Hammersmith & City lines, where passengers can use West Ham station with a short walk along the Greenway to the park. London Underground tickets are being accepted on local bus services."
The service resumed later, but was still running with severe delays, TfL said.
Events at the park on Friday include Jessica Ennis beginning her campaign for gold in the women's heptathlon.
Passengers had to wait for up to 40 minutes at St Pancras for Southeastern's high-speed Javelin services to Stratford International, according to traffic information service Inrix.
Queues snaked down the road outside the station, with some angry customers waiting to get to the Olympic Park taking to Twitter to vent their frustration.
Twitter user Mary Harvey posted a photograph of a long line of people waiting to get into the station, and wrote: "Central line broken. 50000 people to the Olympic park on the javelin = MASSIVE queues. Nice one London transport."
There were also delays of just under an hour on the Docklands Light Railway between Stratford and Canary Wharf this morning because of a signal failure at Poplar, and some traffic problems on the roads around Earls Court due to a burst water main, an Inrix spokesman said.